Aromatics alkylation provides an important commercial route to a number of industrial chemicals. For example, ethylbenzene is a valuable commodity chemical used for the production of styrene and is produced commercially by the alkylation of benzene with ethylene. Similarly, cumene, which is used as a precursor in the production of phenol, is produced commercially by the alkylation of benzene with propylene.
In the past solid phosphoric acid and Freidel Crafts catalysts, particularly aluminum chloride, were used to catalyse the alkylation of aromatic compounds with alkylating agents, such as ethylene and propylene. More recently, however, these conventional catalysts have been increasingly replaced by zeolitic materials. For example, the well known Mobil/Badger ethylbenzene process (see Oil and Gas Journal, volume 7, 1977, pages 58-61) is a vapor phase process for producing ethylbenzene by reaction of ethylene with benzene over the zeolite ZSM-5. Today, other zeolites, most notably MCM-22, are beginning to replace ZSM-5 as the preferred aromatics alkylation catalysts particularly in view of their enhanced selectivity to the desired monoalkylated product and their ability to operate at less severe, liquid phase conditions. For example, U.S. Pat. No. 4,992,606 discloses the use of the zeolite MCM-22 in the alkylation of aromatic compounds with short chain (having 1 to 5 carbon atoms) alkylating agents.
MCM-56 is a layered oxide material, rather than a three-dimensionally ordered zeolite, in which each layer in MCM-56 is porous and has a framework structure closely related to that of MCM-22. MCM-56 and its synthesis are described in U.S. Pat. No. 5,362,697. U.S. Pat. No. 5,453,554 discloses the use of MCM-56 as a catalyst in the alkylation of aromatic compounds with short chain (having 1 to 5 carbon atoms) alkylating agents. As disclosed in FIGS. 6 and 7 of U.S. Pat. No. 5,453,554, MCM-56 offers potential advantages over MCM-22 for the production of ethylbenzene and cumene, particularly under liquid phase conditions, since MCM-56 is a more active alkylation catalyst than its zeolitic counterpart MCM-22. The entire disclosures of U.S. Pat. Nos. 5,362,697 and 5,453,554 are incorporated herein by reference.
MCM-56 has a chemical composition which can be represented by the formula: EQU X.sub.2 O.sub.3 :(n)YO.sub.2
wherein X is a trivalent element, such as aluminum, boron, iron and/or gallium, preferably aluminum, Y is a tetravalent element, such as silicon and/or germanium, preferably silicon, and n is less than 35, typically from 5 to 25, preferably from 10 to 20 and most preferably from 13 to 18. As-synthesized, MCM-56 has the following formula, on an anhydrous basis: EQU (0-2)M.sub.2 O:(1-2)R:X.sub.2 O.sub.3 :(n)YO.sub.2
wherein M is an alkali and/or alkaline earth metal and R is an organic moiety and wherein the M and R components are associated with the material as a result of their presence during the synthesis of the MCM-56.
When MCM-56 is to be used as an aromatics alkylation catalyst, the organic moiety R is removed, normally by calcination, and the alkali and/or alkaline earth metal M, which would otherwise render the material catalytically inactive, is replaced, normally by the hydrogen cation. As disclosed in Example 11 of U.S. Pat. No. 5,453,554, preparation of an aromatics alkylation catalyst from as-synthesized MCM-56 typically involves mixing the material with an inorganic oxide binder, normally alumina, and then extruding the mixture into catalyst particles of the required shape. The resultant catalyst particles are then dried at about 120.degree. C. and then contacted with ammonium nitrate solution so that the alkaline and/or alkaline earth metal cations are at least partially replaced by ammonium cations. The exchanged catalyst particles are rinsed with water, dried and then calcined at about 540.degree. C. to convert the MCM-56 to the hydrogen form and remove the organic moiety R.
According to the invention, it has now been found that, if the ammonium exchange is effected prior to any calcination and catalyst particle formation, the activity of the resultant MCM-56 catalyst is surprisingly and significantly increased as compared to that obtained using the method described in U.S. Pat. No. 5,453,554. While the reason for this enhanced activity is not fully understood, it is believed that the alkali and/or alkaline earth metal becomes locked within the framework structure of the MCM-56 during catalyst particle formation and/or subsequent drying of the catalyst particles, making its subsequent removal by ammonium exchange much more difficult. By effecting the ammonium exchange prior to catalyst particle formation and calcination, the method of the invention avoids this problem.